DARK-GROUND ILLUMINATION 165 



over, some means must be used to reduce the N.A. 

 This is best effected by slipping into the objective a 

 tubular stop ; this stops out the marginal rays and so 

 reduces the angular aperture, and therefore also the N.A., 

 of the lens. Stops for this purpose are supplied by the 

 microscope makers. It is necessary to use a stop as close 

 to the back lens of the objective as possible, otherwise 

 the end in view (the reduction of the N.A.) will not be 

 properly achieved ; hence the use of the tubular stop of 

 proper length, so that the lower end of it comes close to 

 the back lens of the objective. The permissible N.A. is 

 to some extent limited by the intensity of the light 

 employed, for a relatively weak illuminant allows of 

 somewhat larger N.A. being utilised, and we desire to use 

 as large a N.A. as is permissible if fine detail is to be 

 resolved ; hence the statement at the commencement of 

 the section above on illumination, that it is a mistake, as 

 a rule, to have too intense an illuminant for dark-ground 

 work. 



The simplest method of setting up the illuminant and 

 microscope is as follows (after J. E. Barnard l ) : Take 

 a piece of board about 21 in. long by 9 or 10 in. broad and 

 place the illuminating lamp at one end. The lamp should 

 be surrounded with a circular tin screen painted a dead 

 black and having a round hole 2-2 J in. in diameter cut 

 in it at a suitable height, so that the radiant (i.e. the source 

 of light) is situated at its middle or somewhat above 

 the centre, according to the height of the radiant above 

 the base board. In front of the light and fairly close to it 

 place a bull's eye condenser with the flat surface towards 

 the radiant, and adjust it so that the image of the radiant 

 is projected 7-9 in. from the condensing lens. The micro- 

 scope is then placed in position at the other end of the 

 board, so that the image of the radiant falls exactly on 



1 See Special Report Series, No. 19, Medical Research Committee, 1918. 



